Clean up E0764 explanation

compiler/rustc_error_codes/src/error_codes/E0764.md

@@ -1,12 +1,4 @@
-Mutable references (`&mut`) can only be used in constant functions, not statics
-or constants. This limitation exists to prevent the creation of constants that
-have a mutable reference in their final value. If you had a constant of `&mut
-i32` type, you could modify the value through that reference, making the
-constant essentially mutable. While there could be a more fine-grained scheme
-in the future that allows mutable references if they are not "leaked" to the
-final value, a more conservative approach was chosen for now. `const fn` do not
-have this problem, as the borrow checker will prevent the `const fn` from
-returning new mutable references.
+A mutable reference was used in a constant.
 
 Erroneous code example:
 
@@ -19,6 +11,18 @@ fn main() {
 }
 ```
 
+Mutable references (`&mut`) can only be used in constant functions, not statics
+or constants. This limitation exists to prevent the creation of constants that
+have a mutable reference in their final value. If you had a constant of
+`&mut i32` type, you could modify the value through that reference, making the
+constant essentially mutable.
+
+While there could be a more fine-grained scheme in the future that allows
+mutable references if they are not "leaked" to the final value, a more
+conservative approach was chosen for now. `const fn` do not have this problem,
+as the borrow checker will prevent the `const fn` from returning new mutable
+references.
+
 Remember: you cannot use a function call inside a constant or static. However,
 you can totally use it in constant functions: